Why is trend-following only recommended in Bullish market conditions?

Hey,

Often, when researching trend-following strategies such as moving averages or turtle trading, the author makes explicit note of how the strategy is only viable in bullish market conditions. I also hear this same nugget of wisdom repeated on Reddit, and I was wondering if anyone could enlighten me as to why.

Surely most strategies would perform equally well by taking short positions in a bear market?

If market conditions are relatively flat, then I can see how trend following could be ineffective. However, even in this case, would it not be possible to simply trade on a shorter timescale? If an asset has enough volatility in a given timeframe to cover the brokerage fees, then surely there's still money to be had following the swings, even if the asset returns are stationary over a longer timescale?

And finally I'd appreciate someone telling me why I'm a fucking moron for considering this approach in the first place.

All the best!